One of the things that happens is people bringing in old/unused ammo and asking us to dispose of it. Occasionally that includes something like this:
Once upon a time, Daisy decided to try making firearms and one of their ideas was a .22 with caseless ammo
As I recall, to cock it you worked a lever to compress a spring(like a pellet gun); when you fired the spring drove a piston that forced the air through a tiny aperture to compress and heat it, and that ignited the propellant.
It never caught on. Neat idea, but the propellant could break loose from the bullet, and it could be pretty vulnerable to moisture. As I recall this was years before H&K made a real effort to make a working, reliable caseless bullet(bunch of people still are). Now, it's a curiosity, an idea tried that just didn't work out.